You used QuickBooks Attached Documents in your last version of QuickBooks and have attached documents to one or more items or transactions in your Company File.
You upgrade to the newest version of QuickBooks and you find that documents previously attached to items or transactions are no longer accessible.
When you use QuickBooks Attached Documents, an Attach folder is created and stored by default in the same location as your Company File.
The connection between your Company File and your attached documents will break if you move your Company File to new location and do not move the Attach folder to the same location.
Solution: In your new upgraded company file, move the Attach folder to the right location, then restore to the last backup that was created before you upgraded to the latest version of QuickBooks.